Use of wire loop in fence as a proximity detector (?)

[Some Guy]

Have a fence with sections that are 10 ft wide by 8 ft tall. Was
thinking about running 1 (or more) loops of wire around the perimeter
of each section and, with the right electronics, being able to detect
the presence of large objects (ie people) that may come very close to
the fence.

Alternatively, each loop can be 10 ft wide by 4 ft high (from a
detection stand-point, is this size loop better than 10 x 8?).

Now, suppose that the fence is really a concrete wall (average
thickness = 3 inches). Does it matter which side of the wall the loop
is on?

What would be the prinicple of operation? Would the loop form part of
an oscillator circuit, and a nearby body would change the inductance
of the loop thus causing a frequency shift?

Any example circuits for something like this?

 

[Jim Thompson]

Have a fence with sections that are 10 ft wide by 8 ft tall. Was
thinking about running 1 (or more) loops of wire around the perimeter
of each section and, with the right electronics, being able to detect
the presence of large objects (ie people) that may come very close to
the fence.

Alternatively, each loop can be 10 ft wide by 4 ft high (from a
detection stand-point, is this size loop better than 10 x 8?).

Now, suppose that the fence is really a concrete wall (average
thickness = 3 inches). Does it matter which side of the wall the loop
is on?

What would be the prinicple of operation? Would the loop form part of
an oscillator circuit, and a nearby body would change the inductance
of the loop thus causing a frequency shift?

Any example circuits for something like this?

Humans tend to be non-metallic ;-)

You might be better off detecting capacitance changes.

...Jim Thompson

 

[John Woodgate]

What would be the prinicple of operation? Would the loop form part of
an oscillator circuit, and a nearby body would change the inductance
of the loop thus causing a frequency shift?

People are not 'magnetic'. Any change in inductance would be very, very
small. OTOH, a person close to the fence would change the capacitance of
a plate on the fence to ground. You stand a chance of getting that to
work. Google for 'capacitive touch sensors' or a reasonable facsimile
thereof.

 

[Leon Heller]

Have a fence with sections that are 10 ft wide by 8 ft tall. Was
thinking about running 1 (or more) loops of wire around the perimeter
of each section and, with the right electronics, being able to detect
the presence of large objects (ie people) that may come very close to
the fence.

If this is a security application you'd be better off with piezo cable on
the fence to detect if people are climbing over it. Measurement Specialties
makes it.

Leon

 

[Ian Stirling]

People are not 'magnetic'. Any change in inductance would be very, very
small. OTOH, a person close to the fence would change the capacitance of
a plate on the fence to ground. You stand a chance of getting that to
work. Google for 'capacitive touch sensors' or a reasonable facsimile
thereof.

Some are a bit magnetic.
If you crank the current up high enough, you can actually repel the
people from the fence through magnetism.

Unfortunately, at this sort of power level, the wire will explode at
a speed that makes conventional explosives seem a bit boring.
If you overcome this, then you'll generate enough force to rip the wire
out of solid concrete.
Oh well.

 

[Tim Auton]

People are not 'magnetic'.

People are mildly paramagnetic though.


Tim

 

[N. Thornton]

Humans tend to be non-metallic ;-)

You might be better off detecting capacitance changes.

Indeed. Human will have to be very close, ie touching, for any
noticeable C change. Trouble is on a fance there may be c changes as
the wind blows, depending on the construction details.

Suffice it to say its not a standard method of detecting buggers.

Regards, NT

 

[Some Guy]

You might be better off detecting capacitance changes.

Thanks for the various replies.

Someone mentioned that people are "paramagnetic". Was this a joke (ie
paranormal) ?

Anyone know how this device might work:

http://abacusa.com/aom/cap-sen.html#intrusion_Capacitance_Sensor

I also came across electric-field imaging or sensing featuring this
motorola chip:

http://e-www.motorola.com/files/analog/doc/data_sheet/MC33794.pdf

See also:

http://www.qprox.com/background/electrodes.php
http://www.bik.com/ (many web sites link to this page)

You know, I just thought of something. Aren't I asking for a
"whole-body" theremin?

http://theremin.info/modules.php?name=Conteudo&pa=showpage&pid=57

 

[Scott Stephens]

Any example circuits for something like this?

Not inductance, though you might set up a reluctance sensor with a steel
fence to sense autos or ferrous objects.

I've seen schematics in books with alarm circuits. The better ones have
a couple wires separated and mounted atop the fence. The are fed by
oscillators operating at different, unrelated frequencies (often around
200KHz). When an object cause a capacitance change greater in one than
the other, the alarm triggers.

False alarms (rain) don't trigger, because both oscillators are pulled
equally.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

 

[Terry Pinnell]

People are not 'magnetic'. Any change in inductance would be very, very
small. OTOH, a person close to the fence would change the capacitance of
a plate on the fence to ground. You stand a chance of getting that to
work. Google for 'capacitive touch sensors' or a reasonable facsimile
thereof.

One example is my circuit at
http://www.terrypin.dial.pipex.com/Images/Proximity.gif

Although I used this mainly in a variety of touch lamps, I did have
some success experimenting with several alarm applications. Using an
antenna consisting of a length of wire along a garden fence triggered
the circuit OK, but that was in dry conditions over a few hours. I'd
be pessimistic about long term reliability.

 

[Tim Auton]

Thanks for the various replies.

Someone mentioned that people are "paramagnetic". Was this a joke (ie
paranormal) ?

That was me. And it wasn't a joke, it's a real physical phenomenon. If
atoms in the material have dipole moments but are not usually aligned
they will tend to align in the presence of a strong magnetic field,
resulting in a positive magnetic susceptibilities (the material acts
like a regular magnet). When the magnetic field is removed random
thermal motion destroys the alignment. It's not a very strong effect
though, so I don't know if it would be useful in your application -
you'd have to run the numbers (but I suspect not). A search for
"paramagnetism" should find the details.


Tim

 

[Scott Stephens]

Indeed. Human will have to be very close, ie touching, for any
noticeable C change.

I have built HF proximity sensors that detected people several feet away.

I've heard that DC (mosfet) electrostatic proximity sensors can pick up
the charge or field distortion caused by a person 15 feet away, in the
dry desert.

Trouble is on a fance there may be c changes as

the wind blows, depending on the construction details.

Yes, proximity sensors are some of the most falsing sensors.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

 

[Scott Stephens]

Someone mentioned that people are "paramagnetic". Was this a joke (ie
paranormal) ?

Do a web search for "levitating frog"

You know, I just thought of something. Aren't I asking for a
"whole-body" theremin?

Some proximity sensors detect capacitive oscillator pulling. Others
detect capacitive loading, or unbalancing a bridge. And then there's the
ubiquitous "Stud Finder", that IIRC uses a capacitive induced pulse
width change.

There are security and alarm system newsgroups where you could get
practical info.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

 

[Ken Smith]

Humans tend to be non-metallic ;-)

You might be better off detecting capacitance changes.

Or operating at a high enough voltage that the arc discharge loads the
tuned circuit if someone gets too close. :>

The human body is about like salt water. At higher frequencies, some
loading of the Q of the tuned circuit may be seen. The frequency will
shift down slightly due to the capacitance effects. Both effect, I
suspect, would be about equal to the effects from the phase of the moon.

 

[Ken Smith]

Now, suppose that the fence is really a concrete wall (average
thickness = 3 inches). Does it matter which side of the wall the loop
is on?[/QUOTE]


Give up on making anything but a weather gauge is the concrete wall is in
the picture. IMO

You could:

Concrete contains a fair amount of K40 which gives off low energy gamma
rays. A person between the concrete an a radiation detector would block
the rays and casue a dip in the count rate.

 

[Ken Smith]

People are mildly paramagnetic though.

With clothes on, almost everyone is magnetic. Zippers, snaps, rivets,
belt buckles, arch supports, wrist watches and eyeglasses are almost
always magnetic.

 

[dave vanhorn]

You know, I just thought of something. Aren't I asking for a

"whole-body" theremin?

http://theremin.info/modules.php?name=Conteudo&pa=showpage&pid=57

Terpistone is what you're after.

But I think the capacitance change of the fence wobbling about will
swamp any desired signal.

There are also sensors for fence climbing that use Triboelectric wire.
Most all wire is somewhat triboelectric, except that which is
manufactured for microphone cables, where it is expected to be smacked
about and stepped on without causing noise.

 

[Barry Lennox]

Have a fence with sections that are 10 ft wide by 8 ft tall. Was
thinking about running 1 (or more) loops of wire around the perimeter
of each section and, with the right electronics, being able to detect
the presence of large objects (ie people) that may come very close to
the fence.

Alternatively, each loop can be 10 ft wide by 4 ft high (from a
detection stand-point, is this size loop better than 10 x 8?).

Now, suppose that the fence is really a concrete wall (average
thickness = 3 inches). Does it matter which side of the wall the loop
is on?

What would be the prinicple of operation? Would the loop form part of
an oscillator circuit, and a nearby body would change the inductance
of the loop thus causing a frequency shift?

Any example circuits for something like this?

It may not be suitable for what you have in mind, but there's a lot of
security fences that use a standard electric fence energizer to power
the fence, and the voltage pulse is monitored at the return end. A
significant drop means that somebody, or something, has sucked down
the pulse (and got a belt in the process) Widely used in used car
lots, but also a little prone to nuisance alarms by lads shorting out
the fence for the hell of it.

Barry Lennox

 

[N. Thornton]

N. Thornton wrote:

I have built HF proximity sensors that detected people several feet away.

I've heard that DC (mosfet) electrostatic proximity sensors can pick up
the charge or field distortion caused by a person 15 feet away, in the
dry desert.

Trouble is on a fance there may be c changes as

Yes, proximity sensors are some of the most falsing sensors.

So how are you going to detect the tiny capacitance changes from
someone several feet away when the thing is on a fence getting blown
by strong winds, and therefore changing c? Even if it were a stable
stand alone device just near the fence, the fence would still alter c
too much. I ask because youve done it, I havent - I just am not seeing
how it would work.

Regards, NT

 

[budgie]

Have a fence with sections that are 10 ft wide by 8 ft tall. Was
thinking about running 1 (or more) loops of wire around the perimeter
of each section and, with the right electronics, being able to detect
the presence of large objects (ie people) that may come very close to
the fence.

Alternatively, each loop can be 10 ft wide by 4 ft high (from a
detection stand-point, is this size loop better than 10 x 8?).

Now, suppose that the fence is really a concrete wall (average
thickness = 3 inches). Does it matter which side of the wall the loop
is on?

What would be the prinicple of operation? Would the loop form part of
an oscillator circuit, and a nearby body would change the inductance
of the loop thus causing a frequency shift?

Any example circuits for something like this?

There are a number of buried cable systems that use signals in the HF region to
detect above-ground intruders. Senstar made a system named Sentrax which our
local prisons used in the nineties. Maybe there's a start there ...